Semiconductor lasers capable of producing continuous stimulated radiation at wavelengths in the vicinity of 1.1-1.7 .mu.m at room temperature are of interest for communications systems using fiber optics, since it is in this wavelength range that both the transmission losses and the dispersion in high-quality glass fibers are low.
Semiconductor lasers of quaternary III-V alloys of GaInAsP grown on a binary compound of InP (double heterostructures or DH) have proven practical for operation at this frequency range. Furthermore, a particular type of laser construction, i.e., the buried layer type or BH laser for "buried heterostructure" laser; wherein the active layer (GaInAsP) is both vertically and laterally confined (Proceedings of the IEEE, Vol. 64, No. 10, Oct. 76, pp 1528-1529), has been of particular importance for reducing threshold current, I.sub.th, in semiconductor lasers.
As a result of this interest, various techniques for fabricating BH laser diodes have evolved. The most commonly used technique is represented by Hirao et al., "Fabrication and Characterization of Narrow Stripe InGaAsP/InP Buried Heterostructure Lasers", J. Appl. Phys. 51(8) August 1980. In the Hirao et al. technique, a mesa structure is formed by chemical etching such that the active layer is located just above the neck of the mesa. Then, p- and n-type epitaxial InP "burying" layers are regrown on the exposed surfaces of the crystal. This method suffers from the difficulties attendant with epitaxial regrowth, especially over non-planar surfaces. In addition, it is difficult to obtain uniformly etched mesas and the thicknesses of the regrown layers are critical.
I. Mito et al. in "InGaAsP Planar Buried Heterostructure Laser Diode (PBH-LD) with Very Low Threshold Current", Electronics Letters, Vol. 18, No. 1, Jan. 7, 1982, have devised an improved planar BH laser diode structure in which the initial "burying" layers are formed on both sides of the mesa, but not on the mesa top which at this point comprises the active layer. Next, p-InP embedding and n-InP cap layers are grown by the liquid phase epitaxial (LPE) process over this "planar" surface. This technique suffers from the problems of LPE regrowth, especially on the narrow width active layer mesa top; which tends to inhibit proper nucleation for epitaxial growth.
K. L. Yu, et al., in "Groove GaInAsP Laser on SemiInsulating InP", Electronics Letters, Vol. 17, No. 21, Oct. 15, 1981, provides yet another solution to this vexing problem in which a single LPE growth is made on a grooved substrate. While this solution eliminates the need for two LPE growth steps, it requires etching of dovetail shaped grooves and subsequent LPE growth over such non-planar surfaces.
Murotani et al. in "InGaAsP/InP Buried Crescent Laser Emitting at 1.3 um with Very Low Threshold Current", Electronics Letters, Vol. 16, No. 14, July 3, 1980, have devised a structure similar to Yu et al. above, in that LPE is required over etched grooves in order to achieve a BH laser diode. Also, Kishino et al. "Mesa-Substrate Buried-Heterostructure GaInAsP/InP Injection Lasers", Electronics Letters, Vol. 15, No. 4, Feb. 15, 1979, have fabricated GaInAsP/InP DH lasers of the BH type utilizing a single-step epitaxial growth which they call the mesa-substrate buried-heterostructure (m.s.b.) laser. In the m.s.b. structure, the active region is formed by LPE growth over a non-planar mesa substrate.
Lastly, Chen et al., in "Embedded Epitaxial Growth of Low-Threshold GaInAsP/InP Injection Lasers", Appl. Phys. Lett. 38(5), Mar. 1, 1981, discloses a GaInAsP/InP laser device structure formed by single embedded LPE growth through oxide openings in a mask. Again, this technique suffers from the dual problem of (a) the difficulty of inducing proper nucleation on narrow strips of exposed substrate and (b) the resultant non-planar structure produced (as shown in FIG. 2 of Chen et al.).
In addition to the above described fabrication problems, high yield and low threshold current, I.sub.th, have been the goals of those skilled in the art. I.sub.th is related to the width of the active region. The narrower the better. Mito et al., above referenced, reports the lowest CW threshold current, I.sub.th, attained with their structure was 8.5 mA at room temperature and the best laser devices of Murotani et al. (above referenced) operated at "the very low" threshold current of 28 mA CW at room temperature.
Accordingly, it would be highly desirable to obtain a double heterostructure laser diode of the BH type wherein the active layer is uniformly and narrowly defined, conventional epitaxial regrowth on a non-planar surface is eliminated, the yield is high and the I.sub.th is below 10 mA CW at room temperature.